Choreographing the Dance of Traffic Lights

By CATHERINE GREENMAN

Published: September 17, 1998

IN commuter heaven, all traffic lights would magically switch to green for approaching drivers, forming strings of fiery green balls that went on for miles. Pedestrians in paradise would encounter ''Walk'' signs permanently aglow, and drivers would wait patiently as the walkers dawdled the day away crossing the street.

In real life though, drivers inevitably hit red. And in New York City, when they must stop is usually controlled by the intricate computer network governing many of the city's lights, a system based in a single building in Queens, at the Traffic Management Center of the city's Department of Transportation.

How does an engineer at the traffic center tell traffic lights miles away what color lights to flash, and for how long? The center's computers connect to Manhattan traffic lights by sending data through large coaxial cables to signal controller boxes at intersections (the other boroughs are connected via phone lines).

Of the 45,000 intersections in the five boroughs, 10,800 are directed by traffic signals. The center's computers control 6,000 of the intersections via signal controller boxes; each box is wired to all the traffic lights at that intersection. The other 4,800 traffic lights run on timers with preset patterns. If people report a problem with one of these non-networked signals, a traffic center engineer will go out and adjust it manually.

The city is divided into 15 clusters, or neighborhoods; each has its own computer and has 640 to 720 intersections. Manhattan has separate clusters for the East Side, the West Side, uptown, midtown and downtown. Fifteen area computers at the traffic center send information to each cluster of intersections, and these computers are in turn networked to a supervisory computer, which monitors the entire system and coordinates traffic patterns between them.

Every morning, traffic center engineers sit in front of computer terminals and train their eyes on a wall of monitors that display surveillance gathered by 55 closed-circuit cameras located at major traffic arteries. The buzz of a radio stream coming in from police officers on traffic patrols is interrupted several times an hour by the shrill ring of a telephone as one of the several hundred officers or a traffic center official calls in to report a problem in the field. Using that information, the traffic center can respond to different situations and adjust the light patterns.

Traffic lights run on timed cycles that last 60, 90 or 120 seconds. In general, green lights stay on longer for traffic on the avenues in Manhattan and on major roadways in all of the boroughs.

Under ordinary traffic conditions, the area computers controlling each cluster of intersections signal a chain of lights to turn green in 10-second progressions. That gives each light you're driving toward time to turn green if you are driving about 30 miles an hour (which, wouldn't you know, is the New York City speed limit). Under that pattern, you should hit a long series of green lights on major arteries before having to stop.

The traffic center uses the same principle -- but with red lights -- on major arteries to divert traffic away from problem areas. ''When the scaffolding at Times Square fell this summer, our goal was to make it inconvenient for people to get there,'' said Jack Larson, deputy commissioner for operations at the Department of Transportation. ''So we created a red-light pattern up Fifth Avenue and down Broadway, making it preferable to travel on alternate routes.''

To smooth conditions in midtown during the day, the traffic center also creates patterns that favor avenues located away from the hub, like Second and Eighth Avenues.

City traffic would be much worse if the traffic center did not intervene, Mr. Larson said. ''When a major north-south artery in the city is blocked, and the timing isn't modified, the backup just gets longer and longer,'' he said. By holding more traffic back with red lights at a significant distance from an incident, the traffic center gives drivers enough time to go down a side street before getting stuck at the site of the incident. The city's traffic managers try to head off such snarls. ''That,'' Mr. Larson said, ''would not be pretty.''

The traffic center engineers also create patterns via computer to favor traffic on the roads leading into Manhattan during the morning rush period and out of the borough in the evening rush.

From 7 A.M. to 10 A.M. on the Queensboro Bridge, for example, lights give longer green lights for through traffic and shorter greens for side streets, which helps the inbound Manhattan traffic move more quickly. At 10 A.M., the pattern shifts to a midday progression that gives equal amounts of green-light time to the main thoroughfare and the side streets. The programmed pattern shifts automatically again at 3 P.M. to favor the main roadway so traffic can move outbound from Manhattan more smoothly. At 7 P.M., the pattern evens out for the rest of the night.

For pedestrians, the traffic center configures the signals based on an average stride of four feet per second (slow walkers take notice), and it programs each light to flash ''Don't Walk'' for a minimum of seven seconds. Although it is a rare person who is not tempted to break into a mad dash at the sight of this warning, flashing ''Don't Walk'' signals are designed to give you ample time to finish crossing the street -- if you started when the sign said ''Walk.''

Traffic center engineers can also program the light patterns to adapt to different neighborhoods. In areas with lots of elderly people or schoolchildren, for example, the traffic center lengthens the flashing ''Don't Walk'' signal to fit an average speed of three feet per second.

Other patterns have been designed to accommodate things like Presidential visits, parades, water main breaks and road construction. ''Every year, we learn from experience and create new patterns to smooth out the problems,'' Mr. Larson said.

Photo: GROUND ZERO FOR GRIDLOCK BUSTINGS: Engineers at the Traffic Management Center of the New York City Department of Transportation in Queens compare information on their computer screens against the wall map of Manhattan. The green lights on the map indicate major streets or avenues. The center's computers control traffic lights at 6,000 intersections in the city's five boroughs. (Don Hogan Charles/The New York Times) Chart: PC Work Stations Several work stations in the main room at the traffic center allow engineers to adjust traffic signal patterns, based on input they get from the field. Supervisory Computer The supervisory computer has a central database that sends and receives information to and from the area computers. Engineers can view the entire network from this computer. Area Computers Each of the 15 area computers controls between 640 and 720 intersections. There are 5 for Manhattan, 4 for Brooklyn, 3 for Queens, 2 for the Bronx and 1 for Staten Island. These computers also monitor sensor data collected at intersections. Modem This translates high-frequency radio signals, transmitted through coaxial cables and phone lines, into binary code that the area computers can process. Signal Controllers Every signal controller box has its own address; when information addressed to a box is transmitted, the signal controller adopts that pattern at its intersection. Default timers control the 4,800 non-networked signal controllers. If a networked signal controller loses its link to the traffic center, it will automatically revert to a default timer. Magnetic loops embedded into major intersections detect metal from passing cars and report the traffic counts back to the TMC. Timing During a cycle, a green light on the signal facing the street corresponds with a red light on the signal facing the avenue; an amber light on the street signal corresponds with a red light on the avenue signal, and vice versa. There is usually a two-second period when both sides are red, and then the cycle starts again. Inside the Signal Box Inside each signal controller box is a constantly rotating dial and a long bar with up to 27 cams (depending on how many lights are at the intersection) that each control the different light phases. Each circular cam has ridges on it that activate one of three phases for a traffic light. Each full circle of the dial corresponds to a full cycle of traffic lights in an intersection. The cycle length can be set to 60, 90 or 120 seconds. Each time a timing key reaches the top of the dial, it connects the power that advances the cams one step, changing the color of the signal lights. A CLOSE LOOK -- Tracking complicated intersections on computers allows traffic engineers to adjust the flow of vehicles to avoid creating trouble spots. (Source: New York City Department of Transportation)